1. Field of the Invention
The invention relates to methods of manufacturing rotary drill bits, and particularly rotary drag-type drill bits of the kind comprising a bit body having a threaded shank for connection to a drill string and a leading face on which are mounted a plurality of cutters.
The cutters may, for example, be preform cutting elements comprising a layer of superhard material, such as polycrystalline diamond, bonded to a substrate of less hard material, such as cemented tungsten carbide. The substrate of the cutting element may be bonded, for example by brazing, to a carrier which may also be of cemented tungsten carbide, the carrier then being brazed within a socket on the leading face of the bit body. Alternatively, the substrate of the cutter may itself be of sufficient size to be brazed directly within a socket in the bit body.
2. Description of Related Art
Drag-type drill bits of this kind are commonly of two basic types. The bit body may be machined from metal, usually steel, and in this case the sockets to receive the cutters are formed in the bit body by conventional machining processes. The present invention, however, relates to the alternative method of manufacture where the bit body is formed using a powder metallurgy process. In this process a metal mandrel is located within a graphite mold, the internal shape of which corresponds to the desired external shape of the bit body. The space between the mandrel and the interior of the mold is packed with a particulate matrix-forming material, such as tungsten carbide particles, and this material is then infiltrated with a binder alloy, usually a copper alloy, in a furnace which is raised to a sufficiently high temperature to melt the infiltration alloy and cause it to infiltrate downwardly through the matrix-forming particles under gravity. The mandrel and matrix material are then cooled to room temperature so that the infiltrate solidifies so as to form, with the particles, a solid infiltrated matrix surrounding and bonded to the metal mandrel.
Sockets to receive the cutters are formed in the matrix by mounting graphite formers in the mold before it is packed with the particulate material so as to define sockets in the material, the formers being removed from the sockets after formation of the matrix. Alternatively or additionally, the sockets may be machined in the matrix. The cutters are usually secured in the sockets by brazing.
In order to braze the cutters in place the cutters are located in their respective sockets with a supply of brazing alloy. The bit body, with the cutters in place, is then heated in a furnace to a temperature at which the brazing alloy melts and spreads by capillary action between the inner surfaces of the sockets and the outer surfaces of the cutters, an appropriate flux being used to facilitate this action.
During the process of brazing the cutters to the bit body, the bit body must be heated to a temperature which is usually in the range of 500.degree.-750.degree. and with the steels hitherto used in the manufacture of the bit bodies of rotary drag-type bits, the heating/cooling cycle employed during infiltration of the matrix and during brazing of the cutters in position has the effect of reducing the hardness and strength of the steel. In view of this, it has been the common practice to manufacture the steel mandrel of a matrix bit in two parts. A first part is mounted within the mold so that the solid infiltrated matrix may be bonded to it and the second part of the mandrel, providing the threaded shank, is subsequently welded to the first part after the matrix has been formed and after the cutters have been brazed into the sockets in the matrix. The part of the mandrel providing the shank does not therefore have its hardness or strength reduced by the brazing process nor by the heating/cooling cycle of the infiltration process.
It would be desirable to avoid this necessity of welding a separate shank part to the mandrel after formation of the matrix, since this not only adds to the cost of the manufacturing process but the necessity of welding the parts together may compromise the design of the bit body. For example, the bit body must be of sufficient length, and so shaped, as to provide a region where the two parts can be welded together. Accordingly, a one-piece mandrel could be shorter in length than a two-piece body and this may have advantage, particularly where the drill bit is for use in steerable drilling systems.
Clearly, the necessity of subsequently welding a separate shank part to the mandrel of the bit after formation of the matrix could be avoided if the mandrel were to be formed from a material which was not reduced in hardness and strength during the heating/cooling cycle employed during the brazing of the cutters on the drill bit. This would enable the mandrel to be formed in one piece, including a portion to provide the threaded shank of the drill bit.
One type of material which might be used for this purpose is a precipitation hardening alloy, such as a precipitation hardening steel or stainless steel. A characteristic of a precipitation hardening alloy is that it hardens when subjected to an appropriate heating/cooling cycle and it is therefore possible to control the heating/cooling cycle to which the drill bit is subjected during brazing of the cutters on the bit in such a manner as to harden the alloy of the mandrel.
However, alloys of this type have different thermal characteristics from the matrix formed around the mandrel in the manufacture of the matrix drill bit, and a result of this mis-match of thermal characteristics may be a tendency for the matrix to crack either during the cooling of the matrix and mandrel following the infiltration of the matrix, or in the subsequent heating/cooling cycle for brazing the cutters to the bit body.
The present invention sets out to overcome this problem while still permitting the mandrel to include a portion to provide the threaded shank of the drill bit without the necessity of welding such portion to the mandrel after formation of the matrix.